1. Field of the Invention
The present invention relates to a variable radio frequency filter, and more particularly, to a variable frequency band filter capable of varying the resonance frequency band.
2. Description of the Related Art
In general, a business provider of a wireless communication service is allocated a frequency band from, for example, a regulatory body of the country in which the provider resides, and thus can provide general subscribers with service on this frequency band. In the case of a commercial wireless communication service, each service provider is allocated a different frequency band. The service provider may divide the allocated frequency band into a number of channels having predetermined bandwidths, when needed by a communication system, or in order to improve the efficiency of using the frequency.
For example, in the current code-division multiple access (CDMA) mode, this is referred to as FA (frequency allocation), where each channel can have a bandwidth of 1.23 MHz, and a service provider having a bandwidth of 10 MHz allocated to it generally uses seven FAs. In the W-CDMA mode, the bandwidth of one FA is 3.84 MHz. Accordingly, a service provider of a wireless communication service can divide the allocated frequency band into a number of channels and choose one of them as desired. As known in the art, different radio frequency filters are separately manufactured and supplied according to the frequency band of respective service providers of wireless communication services.
A conventional radio frequency filter 100 will now be described with reference to FIGS. 1 to 6.
FIG. 1 is a perspective view showing a conventional cavity filter. As shown, the cavity filter includes a housing 110, disk-shaped resonator rods 120 (see FIG. 4), a cover 160, and tuning/coupling screws 170 and 175. The housing 110 has an input connector 111 and an output connector 113. The interior of the housing 110 is divided into a number of containing spaces by diaphragms 130. The disk-shaped resonator rods 120 are contained in the respective containing spaces.
The input connector 111 and the output connector 113 are positioned on the same side of the housing 110 and each of them is connected to a chosen containing space. The diaphragms 130 have coupling windows 131, 132, 133, 134, and 135 formed therein for serial connection from a containing space, to which the input connector 111 is connected, to another containing space, to which the output connector 113 is connected. The housing 110 has an open upper surface, and after the disk-shaped resonator rods 120 are positioned in the respective containing spaces, the upper end of the housing 100 is sealed using the cover 160.
The disk-shaped resonator rods 120 are composed of resonator rods 121, which extend from the bottom surface of the housing 110, and disks 122, which extend along the upper outer peripheral surfaces of the resonator rods 121 in the diametric direction thereof. The radio frequency filter 100, having disks 122 that are positioned on the resonator rods 120 which are assembled in the housing 110, is characterized in that it is operated for a low resonance frequency.
The interrelationship between the resonance frequency and the housing 110, the disk-shaped resonator rods 120, the diaphragms 130, as well as the cover 160, will now be further explained with reference to FIGS. 1 to 6.
In general, the resonance frequency is determined by values of capacitance and inductance, which are formed among capacitive components 17 and inductive components 19 constituting a resonance circuit formed by housing 110, disk-shaped resonator rods 120, diaphragms 130, and a cover 160, as is clear from the circuit diagram shown in FIG. 6. Referring to FIGS. 4 and 5, the input and output connectors 111 and 113 are connected the disk-shaped resonator rods 120 via an input terminal coupling copper wire 115 and an output terminal coupling copper wire 117, respectively. The resonance frequency of the radio frequency filter 100, configured as above, is affected by the length, outer diameter, and the like of the disk-shaped resonator rods 120 and is tuned more precisely with separate tuning/coupling screws 170 and 175.
Referring to FIG. 1, the tuning/coupling screws 170 are 175 are fastened on the cover 160 at locations corresponding to those of the disk-shaped resonator rods 120, which are contained in the housing 110, as well as at locations corresponding to those of the coupling windows 131 to 135, which are formed in the diaphragms 130. The tuning/coupling screws 170 and 175 are used to tune the resonance and coupling characteristics of the radio frequency filter 100 and are fixed using nuts 171, after the tuning, to prevent them from rotating.
The cover 160 is provided with fastening holes 169 for screws 179, and the housing 110 is provided with fastening tabs 180 on its upper end to fix the cover 160 on the upper end of the housing 110. The tuning/coupling screws 170 and 175 are fastened into screw holes (not shown), which are formed on the cover 160, and are used to tune the resonance frequency, inductance, or capacitance. In other words, the radio frequency filter 100 is tuned by tightening or loosening the tuning/coupling screws 170 and 175 to obtain desired resonance and coupling characteristics.
After the tuning of the radio frequency filter 100 is completed, the tuning/coupling screws 170 and 175 are fixed on the cover 160, for example, using nuts 171, so that the resonance frequency, as well as the resonance and coupling characteristics, will not change due to undesired rotation of the tuning/coupling screws 170 and 175. The tuning/coupling screws 170 and 175 can thus be classified as tuning screws 170, which are fixed at locations corresponding to those of the disk-shaped resonator rods 120 and are used to tune the resonance characteristics, and coupling screws 175, which are fixed at locations corresponding to those of the coupling windows 131 to 135 and are used to tune the coupling characteristics. Accordingly, the tuning/coupling screws 170 and 175 have different roles according to their respective locations.
A dielectric filter is another kind of filter and has the same construction as the cavity filter except that the disks are made of dielectric substance, such as ceramic, having a high dielectric constant and a high Q value, and are positioned in the center of containing spaces. The dielectric filter can have the same resonance frequency and at least the same Q value as in the case of the cavity filter, which is at least twice as large as the dielectric filter, by using disks made of dielectric substance of a high dielectric constant and a high Q value.
In the case of the cavity filter, the diameter and length of the resonator rods and the disks, as well as the distance to the upper side of the housing, are the main factors determining the resonance frequency. In the case of the dielectric filter, the dielectric constant of the disks is the main factor determining the resonance frequency.
However, conventional radio frequency filters, configured as above, are adapted for specific frequency bands or channels. Therefore, they cannot be used for different frequency bands or channels of different service providers. As a result, new radio frequency filters must be manufactured separately for different frequency bands, thus making it very difficult to mass-produce the filters, and also increases the manufacturing cost of the filters.